How is Plaster of Paris Prepared from Gypsum? (With Chemical Equation)
The study of plaster of Paris is important for JEE Main Chemistry because of its wide industrial and medical significance. Plaster of Paris, known chemically as calcium sulfate hemihydrate, is a soft white powder that sets rapidly when mixed with water. It is commonly used in making casts, sculptures, and for various building applications due to its fast hardening property and easy workability.
The chemical formula of plaster of Paris is CaSO4·½H2O, reflecting its hemihydrate nature. It is distinct from gypsum and exhibits unique chemical and physical properties, making it a frequent topic for competitive exams. Understanding its preparation, properties, and uses equips students with essential exam-facing knowledge and connects chemistry to real-world applications.
Plaster of Paris: Formula and Chemical Composition
Plaster of Paris is chemically called calcium sulfate hemihydrate. Its composition is represented as CaSO4·½H2O, meaning each molecule contains one calcium sulfate unit and half a water molecule as water of crystallization. This form of calcium sulfate loses part of its water when heated, forming the dry powder known as plaster of Paris.
| Substance | Chemical Name | Formula | Nature |
|---|---|---|---|
| Gypsum | Calcium sulfate dihydrate | CaSO4·2H2O | Naturally occurring mineral |
| Plaster of Paris | Calcium sulfate hemihydrate | CaSO4·½H2O | Manufactured powder |
Preparation of Plaster of Paris from Gypsum
The preparation of plaster of Paris involves heating gypsum (calcium sulfate dihydrate) at 373 K (about 100 °C). This partial dehydration removes 1.5 molecules of water per formula unit:
- Heat gypsum at 373 K in a dry atmosphere for a limited time.
- Do not exceed 393 K, as this destroys the setting property.
Chemical equation:
CaSO4·2H2O (s) → CaSO4·½H2O (s) + 1.5H2O (g)
Here, CaSO4·2H2O is gypsum, and CaSO4·½H2O is the resulting plaster of Paris. The process is a reversible chemical reaction and exemplifies the concept of water of crystallization.
Properties and Composition of Plaster of Paris
Plaster of Paris is a dry, white, free-flowing powder. Its most important property is its quick setting ability upon addition of water, reforming gypsum and producing a hard mass. This reaction is slightly exothermic and occurs at room temperature.
- Soft, white powder.
- Sets rapidly by absorption of water, reforming gypsum.
- Exhibits expansion on setting, ensuring detailed casts or impressions.
- Non-toxic, but dry powder should be handled with care.
| Feature | Plaster of Paris | Gypsum |
|---|---|---|
| Formula | CaSO4·½H2O | CaSO4·2H2O |
| State (room temp.) | Powder | Solid crystals |
| Solubility in water | Slightly soluble | Very slightly soluble |
| Setting property | Quickly hardens with water | No rapid set |
Uses and Applications of Plaster of Paris
Due to its unique chemical reaction and setting properties, plaster of Paris finds widespread applications in daily life and various fields. For JEE, knowing these real-world examples facilitates smooth revision and problem-solving.
- Immobilizing broken bones (orthopedic casts in medicine).
- Making decorative ceilings, wall finishes, and false ceilings in construction.
- Producing sculptures, statues, and casts in art and crafts.
- Forming molds for pottery and ceramics.
- Used in dental impression materials and laboratory castings.
- As a fire-resistant coating and insulating material in buildings.
For more on everyday chemical compounds, explore Chemistry in Everyday Life. Applications may be directly asked in one-mark exam questions.
Reactions and Setting Mechanism of Plaster of Paris
When water is added to plaster of Paris, it rehydrates and changes back to gypsum. The reaction can be written as:
CaSO4·½H2O + 1.5H2O → CaSO4·2H2O
This setting process is slightly exothermic and responsible for the heat felt during the hardening of the material. The interlocking gypsum crystals form a rigid structure. The ability to reverse this reaction is limited, as the solid mass cannot be powdered again without further heating and loss of water of crystallization.
Key Differences: Plaster of Paris vs. Gypsum vs. Other Plasters
It is crucial for JEE aspirants to distinguish clearly between gypsum, plaster of Paris, and common building plasters. While gypsum is a naturally occurring mineral with more water of crystallization, plaster of Paris is an artificial product made for quick-setting uses. Regular building plaster may also contain lime or cement, which have different chemical properties and uses.
- Gypsum: Natural, does not set quickly, formula CaSO4·2H2O.
- Plaster of Paris: Manufactured, quick-setting, formula CaSO4·½H2O.
- Other plasters: Often include lime (CaO) or Portland cement, used for construction rather than casting.
To strengthen your grasp of chemical formulae and distinctions, check out Compound vs. Mixture and Elements and Atoms Differences.
Summary Table: Revision Notes on Plaster of Paris
| Key Point | Detail |
|---|---|
| Chemical name | Calcium sulfate hemihydrate |
| Formula | CaSO4·½H2O |
| Source | Heated gypsum (CaSO4·2H2O) at 373 K |
| Key reaction | CaSO4·2H2O → CaSO4·½H2O + 1.5H2O |
| Main properties | Quick-setting, expands on setting, forms hard mass |
| Main uses | Medical casts, wall finishes, sculptures, ceramics |
Mastering the differences and chemical principles behind plaster of Paris will help JEE Main students tackle both theoretical and application-based questions confidently. For more concepts and solved problems, browse Vedantu’s Chemistry resources.
FAQs on Plaster of Paris: Chemical Formula, Preparation, and Uses
1. What is the chemical formula of plaster of Paris?
Plaster of Paris has the chemical formula CaSO4·½H2O, known as calcium sulfate hemihydrate.
- It consists of calcium sulfate and a half molecule of water (hemihydrate).
- This formula is important for exams and chemistry concepts.
- It differentiates plaster of Paris from gypsum (CaSO4·2H2O).
2. How is plaster of Paris prepared from gypsum?
Plaster of Paris is prepared by heating gypsum (CaSO4·2H2O) to 373 K (about 100°C), which removes some water of crystallization.
- Chemical equation: CaSO4·2H2O (gypsum) → CaSO4·½H2O (plaster of Paris) + 1½H2O (steam)
- The process is called calcination.
- The preparation involves controlled heating to ensure the hemihydrate form.
3. What is the use of plaster of Paris?
Plaster of Paris is used for its quick-setting and moldable properties in various fields.
- Making casts and bandages for medical applications
- Decorative items and sculptures
- Wall coatings in construction (False ceilings, wall plasters)
- Molds for pottery and industrial prototypes
4. What is the difference between regular plaster and plaster of Paris?
The main difference lies in their composition and setting properties.
- Plaster of Paris is calcium sulfate hemihydrate (CaSO4·½H2O), which sets quickly on mixing with water.
- Regular plaster can refer to gypsum plaster (CaSO4·2H2O) or lime-based plasters, which have different setting rates and uses.
- Plaster of Paris is often used for casting and art; regular plaster is mainly for wall finishing.
5. What is plaster of Paris called in the USA?
In the USA, plaster of Paris is commonly called the same, but may also be referred to as gypsum plaster or quick-setting plaster.
- The chemical composition remains calcium sulfate hemihydrate.
- It is widely available in hardware and craft stores under names like “casting plaster.”
6. Is plaster of Paris safe to handle at home?
Plaster of Paris is generally safe for home use when proper precautions are taken.
- Avoid inhaling dust; use a mask if mixing large quantities.
- Do not allow direct skin contact for long periods, as it may cause irritation.
- Always mix with water in well-ventilated areas and avoid pouring down drains, as it hardens quickly and can block pipes.
7. Can you reverse the setting of plaster of Paris after it hardens?
Once plaster of Paris hardens, the process is not reversible.
- The reaction is chemical and forms a rigid, crystalline structure.
- Heating does not convert it back into usable plaster of Paris.
- Proper mixing proportions should always be used as hardened POP cannot be reused.
8. Why is water of crystallization important in plaster of Paris?
Water of crystallization determines the chemical and physical properties of plaster of Paris.
- Plaster of Paris contains half a molecule of water per formula unit (hemihydrate).
- This partial water content allows it to set into a hard mass when mixed with more water.
- The removal and addition of water control its setting and usability.
9. Is plaster of Paris a pure substance or a mixture?
Plaster of Paris is considered a pure chemical substance, specifically calcium sulfate hemihydrate (CaSO4·½H2O).
- All its properties and reactions are predictable and consistent.
- It is not a mixture, though impurities may be present in practical samples.
10. What happens if you mix extra water with plaster of Paris?
Adding excess water to plaster of Paris delays setting and weakens the final product.
- Extra water reduces hardness and strength.
- Optimally, use the recommended water ratio (roughly 2:1 powder to water by volume).
- Excessive water can also cause cracks and improper binding.
